I unchecked everything related to metrics but the width changed. It should stay the same.
so do I.
I have same problem in multiple master recently.
The File has 2 masters : Bold, Heavy.
Bold master is already filled with all of glyph, and now I’m working on the other master(heavy).
But When I used the paste special, THE WIDTH was changed Though width’s property was same in each master!
Properties of paste special are here.
It is a nightmare. 
That is possible because it does not override the automatic alignment.